162. EFFECT OF TURBULENCE ON MR SIGNAL INTENSITY USING LIMITED FLIP ANGLES AND GRADIENT REFOCUSED ECHOES
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Evans, A J B.A.; Hedlund, L W Ph.D.; Spritzer, C E M.D.; Herfkens, R J M.D.; Blinder, R A M.D. Author InformationKeywords:
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A theoretical description of the nuclear magnetic resonance (NMR) signal from flowing nuclei in refocused gradient-echo pulse sequences, both with continuous- and alternating-phase pulse trains, has been developed. Both laminar and plug flow models have been considered and formulae have been derived that relate mean signal intensity to flip angle, pulse sequence repetition interval (TR), and flow velocity. The degree of signal enhancement or reduction in various conditions of flow and pulse sequences depends on the precise phase relationships between the residual transverse magnetization and each radio-frequency (RF) pulse.
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Gradient echoes techniques have become increasingly popular in clinical practice. The relationship between signal intensity of flowing protons and flip angle in FLASH sequence is described in this paper. Experimental measurements have been performed on a flow phantom in a whole-body NMR imaging system operating at 1.0 T with different radiofrequency pulse flip angles and flow rates. The result indicates that the flowing NMR signal decreases as stimulating flip angle increases in the low velocity ranging up to the maximum intensity at V=L/TR. This relationship is very important in clinic.
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Abstract An in vitro study was performed to investigate the effects of B o inhomogeneity on magnetic resonance images of flow. Controlled inhomogeneity gradients (G 1 ) were applied and the magnitude of the artifacts produced was quantified for different echo delay times ( TE ). Both steady and pulsatile flows were examined. In the presence of an inhomogeneity gradient, signal loss is apparent if the flow is pulsatile and/or if the slice thickness is large. The signal loss increases with increasing TE and G 1 . With pulsatile flow, ghosting artifacts are also generated. These increase in intensity with increasing TE and G 1 . In vivo , field inhomogeneity due to susceptibility variations is large enough to produce these effects. Representative time‐of‐flight images obtained of a normal volunteer with two different TEs demonstrate the effect in vivo . Flow‐related signal loss and artifacts, therefore, increase with increasing TE independent of the moments of the applied gradients.
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It is demonstrated that the signal intensity in a RARE (TSE, FSE…)-sequence with low refocusing flip angle α can be significantly increased by setting the flip angle of the first refocusing pulse to 90°+α/2. In addition to the gain in signal intensity, the initial signal modulations over the first few echoes are reduced compared to a CPMG-echo train with constant α. Magn Reson Med 44:983–985, 2000. © 2000 Wiley-Liss, Inc.
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: To develop suitable strategies for quantification of longitudinal relaxation time (T1) by means of ultrashort echo time (UTE) sequences and the variable flip-angle approach in materials and tissues with extremely fast signal decay.: A recently published modified Ernst equation, which correctly accounts for in-pulse relaxation of transverse magnetization, was used to numerically determine optimal flip angles for reliable assessment of T1 in case of extremely short effective transverse relaxation time (T2*). Various ratios of repetition time (TR) to T1 and radiofrequency (RF) pulse duration (TRF) to T2* were evaluated. Theoretical considerations were applied to solid polymeric material (T2* = 0.295 milliseconds), and T1 quantification was performed using various optimized flip-angle approaches at different RF pulse durations (TRF = 0.1-0.4 milliseconds). Furthermore, in vivo measurement of T1 in cortical bone was exemplarily performed in 3 healthy volunteers to test the applicability of the proposed method in vivo. For in vitro and in vivo studies, MR imaging was performed on a 3 T whole-body MR system using a 3D UTE sequence with a rectangular excitation pulse and centric radial readout.: Optimal flip angles were shown to be strongly dependent on TR/T1 and TRF/T2* ratios. Exemplarily, longitudinal relaxation time of the investigated solid polymeric material was determined to T1 = 223.1 ± 3.1 milliseconds with RF pulse duration of TRF = 0.2 milliseconds, and 12 acquired flip angles ranging from 5 to 60 degrees. Using only 2 optimized flip angles (8 degrees, 44 degrees), T1 of the same material was determined to T1 = 223.8 ± 4.2 milliseconds in a markedly less acquisition time. In vivo evaluation of cortical bone was feasible and showed T1 values of 80.4 ± 25.1 milliseconds, exemplarily.: Using the modified Ernst equation, it seems possible to rapidly evaluate 3D distribution of longitudinal relaxation time in materials and tissues with extremely fast signal decay by means of UTE sequences and only 2 measurements with optimized flip angles.
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